The present invention relates to a communication signal receiver with a plurality of components arranged along at least one signal path, where the components along the or each signal path comprise a filter for processing a received signal and averaging means for deriving a mean value of the signal. The invention also relates to a method of operating a communication signal receiver with a plurality of components arranged along at least one signal path, wherein a received signal is converted to a digital form and the signal is processed by at least one digital filter comprised in said plurality of components.
Communication signal receivers as set out above are used in e.g. wireless telecommunication devices, such as mobile telephones.
A generic communication signal receiver according to the prior art is shown in FIG. 4. The receiver is a homodyne receiver comprising dual communication channels, which are commonly known as I and Q channels. The dual-channel homodyne receiver of FIG. 4 is of a kind, which is frequently used in contemporary digital mobile telephones, such as GSM, DCS or PCS telephones.
The receiver comprises an antenna 400 for receiving an incoming electromagnetic communication signal, such as a TDMA signal (xe2x80x9cTime Division Multiple Accesxe2x80x9d) representing a stream of digital data symbols, which have been modulated onto two orthogonal carrier waves. The received signal is fed through a bandpass filter 402, amplified in an amplifier 404 and then split into two identical signals in a splitter 406. The first of these signals goes to a first signal path, where it is initially mixed in a mixer 420a with an intermediate frequency signal. The intermediate frequency signal is fed from a local oscillator 410 and passes unmodified through a phase shifter 408. Similarly, the second signal goes to a second signal path, where it is mixed in a mixer 420b with the intermediate frequency signal from the local oscillator 410, once the phase of the intermediate frequency signal has been shifted by 90xc2x0 in the phase shifter 408.
The output of the mixer 420a is filtered by a lowpass filter 422a and amplified in a second amplifier 424a. Subsequently, the signal is fed to an AD converter 426a for sampling the signal and converting it to a digital signal comprising aforesaid stream of data symbols. The digital signal is filtered in a digital lowpass filter 430a, and the digital data symbols contained in the signal are supplied, at a node 432a, to a digital memory 450. An average calculator 439a determines the mean value (DC level) of the digital signal and supplies the mean value to a negative input of an adder 440a. At a positive input the adder receives the digital data symbols, that were previously tapped from the signal path at node 432a. Thus, the adder 440a will in effect subtract the signal mean value, as determined by the average calculator 439a, from the digital signal.
Consequently, the output of the adder 440a at the end of the first signal path will finally provide a first part of the stream of digital data symbols, that were contained in and carried by the analog signal initially received at the antenna 400. Correspondingly, the second signal path, starting with a mixer 420b and ending with an adder 440b, will provide a second part of the stream of digital data. The stream of digital data symbols are subsequently used by other components in the mobile telephone for producing e.g. an audible output through a loudspeaker, such as speech from a party with which the user of the telephone is currently having a telephone conversation. Alternatively, the stream of digital data symbols may represent data messages sent between two computers during a data communication session.
The use of digital filters 430a-b introduces a delay in the signal path, due to the inherent operational properties of digital filters, such as FIR (Finite Impulse Response) or IIR (Infinite Impulse Response) filters. Ideally, once the last wanted digital data symbol has been sampled, the components prior to the digital filters (such as the mixers 420a-b, the amplifiers 404a-b, 424a-b and the AD converters 426a-b) should be switched to a passive or idle mode, in order to preserve power and/or to enter a transmit mode as soon as possible.
However, switching off the receiver circuits immediately after the last wanted data symbol has been sampled causes transient noise from the digital filter, due to the rapid change in the DC level of the signal. Therefore, in order to avoid such generation of noise, it has been necessary for prior art receivers, like the generic one described above, to remain active for a certain period of time after the last wanted data symbol. By doing so, the digital filters are fed with a signal with essentially nonvarying DC-level for as long as it takes for the last symbol to pass through the digital filters. Obviously, this contradicts the above objective of allowing an immediate switch to a passive mode or transmit mode.
Therefore, it is an objective of the present invention to overcome the drawbacks of the prior art approach set out above. In particular, the purpose of the invention is to provide an improved communication signal receiver of the type having a plurality of components arranged along at least one signal path, the components along the or each signal path comprising a filter for processing a received signal and averaging means for deriving a mean value of the signal, where the improvements particularly lie in a more rapid switch from active to passive mode and reduced power consumption.
These objectives are achieved by connecting an output of the averaging means to a component located prior to the filter along the signal path, so that a mean value (or DC level) of the signal may be selectively fed back to the filter for a period of time immediately following the sampling of the last wanted data symbol. Since the filter will be supplied with a signal with no rapid change in DC level, the receiver circuits may be immediately switched off, after the last wanted data symbol has been sampled, without generating transient noise in the digital filter.
The objectives are also achieved by a method of operating the communication signal receiver, wherein a received signal is converted to a digital form and the signal is processed by at least one digital filter, by determining an average value of the signal, detecting an event in the signal, and in response thereof feeding the digital filter with the average value.
Other objectives, advantages and features of the present invention appear from the following detailed description, from the attached patent claims as well as from the drawings.